Recently, with the expansion of services dealing with large capacity contents such as audio and video, there is an increasing need for an optical transmission network supporting large capacity and long distance. In order to satisfy such a need, introduction of WDM (Wavelength Division Multiplexing) technology in a recent optical transmission network is being considered.
In the optical communication using the WDM technique, it is possible to form signal channels for respective wavelengths within one optical fiber cable in view of the fact that an optical signal at a certain wavelength passing through the optical fiber does not interfere with an optical signal of a different wavelength.
It is therefore possible to greatly increase the amount of information that can be transmitted per unit time by a single optical fiber cable. In addition, in order to construct a more flexible network, development and introduction of ROADM (Reconfigurable Optical Add/Drop Multiplexer) and the like as a terminal device, a relay device, and an add/drop multiplexer for adding and dropping optical signals in units of optical wavelength are being developed.
Network technology using this ROADM is disclosed in Patent Literature 1 to 4. In ROADM disclosed in the below-listed documents, optical signals transmitted from a connection destination are attenuated, a difference in power between the optical signals is adjusted, and the adjusted optical signals are multiplexed.